Natural gas is generally sold by volumetric measurement, and one way to measure the flowing gas is by passing it thorugh an orifice of a fixed, predetermined size in order to measure the pressure drop across the orifice. An orifice meter or fitting comprises a housing with flow passages in which a disc with an orifice is supported. The housing, which is installed in a gas pipeline, is provided with pressure taps for measuring pressures immediately upstream and immediately downstream of the orifice. Because the orifice plate is subject to wear by impacting of sand, line scale and other foreign particles in the flowing stream, it must be replaced at frequent intervals to insure accuracy in measurement.
Existing orifice fittings generally have housings of the gate valve type containing a carrier plate in which the orifice is secured. The valve body hubs have an outer diameter substantially equal to that of the pipeline but, for any given norminal pipeline diameter there may be two or more inner diameters required, depending upon a number of factors which must be considered in gas metering. This generally requires a manufacturer to stock orifice meters without finishing the flow tubes, and then to complete the tubes to prescribed internal diameter and length upon receipt of a customer's order.